The invention relates to a power supply for electrically drying transformer parts.
When manufacturing transformers, it is essential to dry so-called active parts before the transformers are sold or put into operation. Active parts of this kind are, for example, electrical insulation of the transformer windings which were impregnated in oil or in chlorinated hydrocarbons or were moistened with water for certain reasons.
The transformer parts can be dried from the outside by producing and exposing the windings to warm air. However, it is also possible to dry the windings by using the heat generated by an electrical current which is sent through the windings.
When a transformer is electrically dried, a chamber is provided for receiving the transformer to be dried. The temperature and pressure of the chamber is automatically controlled and the high voltage windings of the transformer are supplied power from an electric power source. The low voltage windings of the transformer are short-circuited such that the short-circuit current dries the transformer windings.
A regular distribution transformer can be dried in approximately six hours. This means that the entire drying process can be carried out within the limits of an 8-hour work day. When the electric power is used for drying the insulation, only a small amount of the energy is used for heating the core, the surrounding air and the container. The electrical method requires only approximately one third of the energy necessary for conventional drying processes. During these conventional drying processes it often occurs that a cloud of oil vapor escapes into the building when the drying chamber is opened once the drying process is completed. This disadvantage is avoided when the electrical method is used.
It is already known to carry out electric drying by applying 20 Hz. voltage to the high voltage winding of the transformer and short-circuiting the low voltage winding (cf. "Transformer Drying Equipment", a brochure of National Industri, Drammen, Norway). In this example, the low-frequency voltage is used to sufficiently reduce the short circuit voltage so as to avoid flashover in the transformer when the pressure is decreased during the drying process. This known method permits the simultaneous drying of several transformers with ratings which vary from 30 to 2000 kVA. All drying variables such as power, voltage, pressure, oil level, and time are controlled by a programmable logic controller. The drying process itself is divided into three cycles. During the first cycle the transformer is heated under normal air pressure to 110.degree. C. so as to achieve a heat transfer to the insulating parts. Subsequently, in a second cycle the transformer is heated to 150.degree. C. and, simultaneously, air is evacuated to 30 mbar. The evacuation will evaporate as much water as is necessary to expel the oxygen from the tank; however, the pressure must be high enough to prevent flashovers in the transformer. In a third cycle the evacuation is continued down to less than 0.5 bar, without heating.
A disadvantage of this known electric drying method is that a 20 Hz generator, including a downstream regulating transformer, are required as a power source. The regulating transformer only permits adjusting the voltage applied to the transformer.